Character String Literals

Introduction

Character String Literals are used to declare text and character data in a program.

String Literals

A String Literal is a sequence of characters often used to convey a message. For example:

Example	Comment
'Pressure: 100 mbar'	A message informing us the pressure is 100 mbar.
''	An empty string.

Strings can be single byte or double byte:

Single byte strings (STRING) are encoded as UTF-8 and enclosed in two apostrophe (') characters.
Double byte strings (WSTRING) are encoded as UTF-16 and enclosed in two quotation mark (") characters.

Note: Both types of string can encode any Unicode character.

Character Literals

A Character Literal is a single element of a character string. For example:

Example	Comment
'A'	The letter 'A'.

Characters can be single byte or double byte:

A single byte character (CHAR) is enclosed in two apostrophe (') characters.
A double byte character (WCHAR) is enclosed in two quotation mark (") characters.

Note: A character can only encode the subset of Unicode characters that fit into a single element. A single byte character (CHAR) can encode any character in the range U+0000 to U+007F. A double byte character (WCHAR) can encode any character in the range U+0000 to U+FFFF.

String Encoding

Single byte strings (STRING) are encoded using UTF-8. Each character is represented by a sequence of 1 to 4 bytes. Double byte strings (WSTRING) are encoded as UTF-16. Each character is represented by 1 or 2 words.

Examples of encoding:

Character	Code Point	STRING Encoding	WSTRING Encoding
Capital Letter A	U+0041	41h	0041h
Cyrillic Letter П (Pe)	U+041F	D0h 9Fh	041Fh
Chinese character 屋 (house room)	U+5C4B	E5h B1h 8Bh	5C4Bh
Linear B Syllable DA	U+10005	F0h 90h 80h 85h	D800h DC05h

Escape Sequences

The dollar sign ($) is used as an escape character to insert any character code into a string.

Sequence	Usage	Description
$$	Both	Dollar sign ($) character
$'	Single	Apostrophe (') character
$"	Double	Double quotation mark (") character
$L	Both	Line feed (U+000A) character
$N	Both	Line feed (U+000A) character
$R	Both	Carriage return (U+000D) character
$T	Both	Horizontal tab (U+0009) character
$nn	Single	Single byte character, where nn is the two digit hexadecimal value of the character
$nnnn	Double	Double byte character, where nnnn is the four digit hexadecimal value of the character

Escape sequence characters after the dollar sign ($) are not case sensitive. You can also use $l or $n for line feed (U+000A), and $r for carriage return (U+000D).

Using Type Prefixes

Character string literals may be declared with a type prefix:

Prefix	Explanation
CHAR#	Declares a single byte character
WCHAR#	Declares a double byte character
STRING#	Declares a single byte string
WSTRING#	Declares a double byte string

Examples

String	Explanation
''	An empty single byte string
""	An empty double byte string
'Pressure Normal$r$n'	The message "Pressure Normal" as a single byte string terminated with a carriage return and line feed
WSTRING#'Pressure Normal$r$n'	The message "Pressure Normal" as a double byte string terminated with a carriage return and line feed. Note: The apostrophe declares a single byte string, but the WSTRING# prefix overrides it to a double byte string.

Standards Compliance

Table 5 & 6 - IEC 61131-3 Second Edition.

Table 6 & 7 - IEC 61131-3 Third Edition.

Further Information

String Functions

To learn about functions that operate on strings.

Common Elements

To learn about other language Common Elements.

Character Codes

For a list of all Unicode character codes used in IEC 61131-3 code.

Glossary

For the meaning of terms used in Fernhill SCADA.